Tires bearing rubber tread have been used for over one century. Wet traction performance of a tire is highly desirable. Due to the many complex factors involved, such as deformation of tread rubber induced by road surface asperities, rate of water drainage between tread rubber and road surface, and possible adhesive interactions at the interface between rubber and road, a quantitative mechanistic understanding of wet traction suitable for rational design of tread compounds is currently absent. Thus, in the pursuit of new rubber materials for further improvement of tire wet performance, scientists and engineers are continuing to seek better physical understanding on different contributing factors to wet traction.
There exist several contributing factors to the total sliding friction of rubber compounds on a wet rough surface. It is believed that a major contribution comes from the hysteretic loss during the high frequency bulk deformation of rubber compounds induced by the multi-scale asperities on the road surface. Conventionally, loss tangent (tan δ) measured at 0° C. (or at some low temperature) and at an appropriate frequency has been employed as the predictor to rank the contribution from this source. It is also likely that adhesion at the interface between rubber compounds and road surface further enhances wet skid resistance. With this in mind, it may be desirable to increase the adhesion between filler particles exposed on rubber surface and the road surface. For the ideal case of one single spherical filler particle in static contact with a perfectly smooth flat surface in water, Europhys. Lett. 52 (5) 551-556 by Papastavrou et al teaches that the adhesion force may be given by the formulaFadhesion=3/2πRWFillerWaterRoad where R is the radius of the sphere and WFillerWaterRoad is the work of adhesion:WFillerWaterRoad=γFillerWater+γRoadWater−γFillerRoad where γFillerWater is the surface free energy of filler particle in water, γRoadWater is the interfacial free energy of road surface in water, and γFillerRoad is the interfacial energy between filler and road surface. It is believed that a high pressure is necessary to bring the filler particle into direct contact with the road surface.
Therefore, one possibility of increasing adhesion between filler particles on the rubber compound surface and road surface is to choose filler particles exhibiting high γFillerWater. It is generally known that γFillerWater is significantly higher for a particle in crystalline phase than that for a particle in amorphous phase as suggested by Lasaga in Kinetic Theory in the Earth Sciences (1998)
U.S. Pat. No. 6,512,038 teaches rubber compositions including amorphous aluminum hydroxycarbonate, amorphous aluminum hydroxyoxycarbonate, or amorphous aluminum oxycarbonate. These compositions can be used in the manufacture of tire treads. The amorphous aluminum fillers generally have a BET specific surface area of between 40 and 150 m2/g. The amorphous aluminum filler is preferably employed in lieu of precipitated silica filler, although it may be used in conjunction with a minor amount of precipitated silica. No mention is made of carbon black as a reinforcing filler. The Examples show higher tan δ at 0° C. than the comparative without the amorphous aluminum filler, and also shows higher tan δ at 0° C., thereby suggesting better wet traction, when the amorphous aluminum filler has a larger BET specific surface area (75 m2/g yielding higher tan δ at 0° C. than 58 m2/g). It is known in rubber industry that when different types of fillers are employed for rubber compounding, such as using precipitated silica to replace carbon black, the conventional predictor tan δ at 0° C. fails to rank the actual wet skid resistance of rubber compounds.